This invention relates to hydraulic piston assemblies and methods for manufacturing piston assemblies.
Piston assemblies are designed and manufactured to be slidably movable within a cylinder. Most piston assemblies include seal and wear rings which are disposed or snapped into annular grooves which are machined into the piston component. The seal ring or rings prevent the flow of a fluid, such as a hydraulic fluid, past the piston as the piston slides within the cylinder. The wear ring or rings carry the bearing loads of the piston.
The prior art piston assemblies typically include an integral steel piston to provide the requisite strength characteristics with bearing material disposed annularly about the piston. The bearing material must provide a given bearing surface for the particular environment in which the piston assemblies are utilized. The amount of bearing surface required determines the length of the piston which, in turn, determines the length of the cylinder to provide a given stroke, i.e., piston movement along the cylinder. Presently most bearing materials comprise a snap-in bearing ring or similar bearing arrangement requiring a length of piston sufficient to receive and lock the bearing material in position. In addition, the piston length must be sufficient to accommodate a seal assembly. A further problem is that when the piston components, including the bearing rings and seals, are assembled onto the steel piston it is difficult to maintain the desired tolerances whereby the components are all sufficiently concentric with a given or central axis of the piston assembly.
Typical prior art piston assemblies are illustrated in U.S. Pat. No. 1,802,281 granted to J. M. Shimmer on Apr. 21, 1931; U.S. Pat. No. 2,984,895 granted to C. F. Griffin, Jr. et al on May 23, 1961; and U.S. Pat. No. 3,463,058 granted to H. W. Rockwell on Aug. 26, 1969.
The patent to Shimmer discloses a piston member having a resilient sleeve stretched over the piston and mechanically retained in place by grooves. The problem with such an assembly is that the nature of the resilient sleeve itself prevents its outer surface from being concentric within the tolerances required for a precision hydraulic cylinder and piston assembly, i.e., since the sleeve is resilient, its outer surface is not stable or fixed.
The Griffin patent discloses a piston assembly having bearing material disposed thereabout in grooves and machined to provide a bearing surface; however, the nature of the assembly is such that the piston must be substantially longer in length than the extremities of the bearing material for accommodating and retaining the bearing material.
The Rockwell patent discloses a piston assembly having two components making up the piston and defining an annular groove for receiving bearing rings, between which is disposed a seal assembly. The problem with such an assembly is that the piston must be of substantially greater length than the bearing material and because of the number of components it is difficult to maintain the tolerances whereby all of the components are sufficiently concentric to a given axis of the piston.
The subject invention overcomes the problems of the prior art assemblies by providing a piston assembly wherein the piston may be of shorter length yet provide the requisite bearing surface and wherein the problem with concentric tolerances is overcome.
Such is attained by a piston assembly constructed in accordance with the instant invention to include an integral piston having end faces with a peripheral surface extending between the end faces and bearing means disposed completely annularly about the piston. The bearing means extends entirely between the end faces for providing a bearing surface adapted for sliding engagement with the inner surface of the cylinder. The piston assembly also comprises an annular groove extending through the bearing means and into the piston between the end faces thereof. The bearing means and the groove are substantially concentric to the central axis of the piston assembly.
The invention also includes a method for manufacturing such a piston assembly by applying a fluid bearing material to the curved cylindrical surface of the piston completely between the end faces thereof, allowing the fluid bearing material to harden, and machining the hardened bearing material completely between the end faces of the piston. In addition, the annular groove is machined through the bearing material and into the piston. The machining is accomplished by machining relative to an axis of the piston to attain the desired concentricity.